The human hip joint comprises a socket or acetabulum and a ball or femoral head. The femoral head is joined to the femur by a neck of reduced diameter and is received within the acetabulum for universal pivotal movement.
If the femoral head becomes diseased or damaged, it can be replaced with a prosthetic femoral component. The surgery includes removal of the natural femoral head and neck, and the implantation of the femoral component in the intermedullary canal of the femur. The femoral component includes a prosthetic femoral head which can cooperate with an acetabular component which may be the acetabulum, or in the case of total hip replacement, an acetabular cup.
In many cases, the diseased region of the femoral head is essentially along the surface, and the subsurface bone tissue is essentially healthy. In these situations, it is undesirable to remove the healthy portions of the femoral head and neck. Accordingly, in these cases, the diseased portion of the femoral head is removed to leave a natural femoral remnant, and a hollow femoral ball or shell is cemented onto the femoral remnant.
This construction, which is known as surface replacement, has the advantage of preserving the maximum amount of bone tissue and preserving the integrity of the intermedullary canal. In addition, it provides excellent bone stock in case of conversion surgery with a stem-type femoral prosthesis. This is a most conservative procedure which can last a long time.
Because the neck diameter varies from patient to patient, the diameter of the femoral remnant will also vary from patient to patient. Accordingly, the femoral shell must be provided with a wide range of inner diameters. The outer surface of the femoral shell must also be provided in a wide range of diameters to cover the anticipated diameters of the acetabulums and the diameters of acetabular cups that might be employed. Consequently, for each possible diameter of femoral remnant, a large number of femoral shells with different outer diameters must be provided if this procedure is going to be used by a wide range of patients. This will amount to an enormous number of prosthetic component combinations that must be held in inventory. The cost to do this is very high.
Another problem with surface replacement is that the femoral shell is cemented onto the femoral remnant. To allow a space for cement, more bone must be removed. The cement may cock the femoral shell, and it can be difficult to obtain a cement layer of even thickness.
If the acetabulum becomes diseased, it may be necessary to affix an acetabular cup within the natural acetabulum. If this should occur, it will also be necessary to replace the prosthetic femoral shell previously cemented onto the femoral remnant. When this becomes necessary, the remnant and the natural femoral neck are removed, and a stem-type prosthetic femoral component is inserted into the intermedullary canal as described above. Accordingly, if this should occur, the advantages of surface replacement are lost.